I agree, fully and it's a really good point that gets overlooked or not spoken about often enough. Songwriting & composing takes many forms and we’re led to believe that unless the artist is the sole ‘songwriter’ on the credits of their songs, it means they have a team doing the work for them, and they have no input.
In some cases that may be true, like some of the examples you gave. However for the greatest artists of all times it’s almost never this simple, and quite frankly it’s ignorant to continue that belief.
Especially since due to a change in laws (I think in the 80s) songwriting credits have to include everyone who had any input in a song, even if they suggested a phrase, a chorus, or were sampled in anyway, they have to be credited - and since song creation is almost never an isolated process, you end up with most songs having more than 1 person credited, even if the other 5 people had very little input, the average person will see 8 people credited with writing a song and assume the artist can't write their own songs by themselves.
Plus like you said, some of the greatest songs have been changed by the artist in ways such as pitch, tone, arrangements, tempo etc and a small creative change can ruin/improve a song tremendously. It's one of the reasons why some artists have gotten signed just off cover renditions they did on already popular songs, but they've changed it in a way to sound unique &/or better than the original artist did. And on the flip side, you can see just how much someone struggles to recreate a song in the same way as the original artist.